The genus Pestivirus of the family Flaviviridae conventionally consists of classical swine fever virus (“CSFV”), Border disease virus (“BDV”), and bovine viral diarrhea virus (“BVDV”). Genomes of several BVDV, BDV and CSFV strains have been sequenced, individual pestiviral proteins have been expressed and viruses derived from (full-length) DNA copies of the RNA genome of BVDV and CSFV have been generated (Renard et al., 1987, EP application 0208672; Collett et al., 1988, Virology 165, 191-199; Mendez et al., J. Virol. 72:4737-4745, 1988; Deng and Brock, 1992, Virology 1991, 865-679; Meyers et al., 1989, Virology 171, 555-567; Moormann et al., 1990, Virology 177, 184-188; Meyers et al., 1989, EP 89104921; Moormann and Wensvoort, 1989, PCT/NL90/00092; Moormann and Van Rijn; 1994, PCT/NL95/00214; Ridpath et al., 1997, Virus Res. 50: 237-243; Becker et al., 1998, J. Virol. 72:5165-5173, Meyers et al., J. Virol. 70:1588-1595, 1996).
The pestivirus genome is a positive-stranded RNA molecule of about 12.5 kilobases containing one large open reading frame (“ORF”). The ORF is translated into a hypothetical polyprotein of approximately 4,000 amino acids, which is processed by virus- and cell-encoded proteases. The ORF is flanked by two conserved nontranslated regions, which are probably involved in the replication of the genome. The 5′-noncoding region also plays a role in initiation of translation.
The polyprotein which is co- and post-translationally processed by cellular and viral proteases contains all the viral structural and nonstructural proteins (for review, see, C. M. Rice: In Fields Virology, Third Edition, 1996 Flaviviridae: The Viruses and their Replication: Chapter 30: pp. 931-959). The viral structural proteins, the capsid protein C and the envelope proteins Ems, E1 and E2, are located in the N-terminal part of the polyprotein. The nonstructural proteins, including the serine protease NS3 and RNA replicase complex NS5A and NS5B, are located in the C-terminal part of the polyprotein.
Pestiviruses are structurally and antigenically closely related. To date, pestiviruses such as BDV, BVDV and CSFV have been isolated from different species, most notably from ruminants and pigs, but infection of humans has also been reported. All pestiviruses have in common the ability to induce congenital infections of fetuses when a pregnant animal is infected. Such fetal infections occur via transplacental infection if the dam undergoes an acute infection during pregnancy or is persistently infected with a pestivirus (Oirschot, J. T. van, Vet. Microbiol. 4:117-132, 1979; Baker J. C., JAVMA 190:1449-1458, 1987; Nettleton P. F. et al., Comp. Immun. Microbiol. Infect. Dis. 15:179-188, 1992; Wensvoort G. and Terpstra C., Res. Vet. Sci. 45:143-148, 1988).
Currently, modified-live, killed and subunit pestivirus vaccines are available. Live-virus vaccines have the advantage over the other types of vaccines of achieving higher levels of immunity without the need of booster vaccination. However, disadvantages include the ability of vaccinal strains to cross the placenta and induce all known consequences of fetal pestivirus infection (Liess B. et al., Zentralblad Veterinarmed. [B] 31:669-681, 1984). Furthermore, modified-live pestivirus vaccines have been reported to cause immunosuppressive effects, probably due to their ability to spread through the vaccinated animal and replicate for several days in lymphocytes and neutrophils, thereby causing leukopenia and horizontal spread (Roth J. A. and Kaeberle M. L., Am. J. Vet. Res. 44:2366-2372, 1983). Furthermore, epizootics of mucosal disease (a consequence of a persistent BVDV infection) and of acute BVDV infections have been reported after vaccination with live-virus vaccines (Lambert G., JAVMA 163:874-876, 1973).
Thus, despite the fact that live vaccines are generally considered as having the best immunological properties, there are distinct downsides to using a live pestiviral vaccine in the control and eradication of pestivirus infections.
These downsides are related to the fact that a conventional live pestiviral vaccine, after inoculation of the animal with the vaccine, undergoes several rounds of replication and spreads through the vaccinated animal. For one thing, this may result in the above-reported shedding of the virus (horizontal spread), which, after all, is a normal result of any viral infection, whereby an animal is infected with a virus after which the virus replicates, spreads through the body, may replicate again, and eventually is shed from the infected animal to spread to and infect a second, contact, animal.
Even more serious, however, are congenital infections with pestiviruses, causing the so-called vertical spread. Fetuses get infected when the virus spreads through the body of a pregnant animal and the virus crosses the transplacental barrier. Depending on the time of gestation and the virulence of the infecting virus, several effects can be noticed. Severe effects include the death of embryos or fetuses, malformations, mummification, stillbirth or perinatal death (Liess B., Vol. 2 Disease Monographs (E. P. J. Gibss, Editor) Academic Press, London. pp 627-650, 1982). Less virulent virus infections, or infections later in gestation, generally result in the birth of congenitally infected offspring (van Oirschot J. T. in: Classical swine fever and related viral infections, B. Liess (ed) Martinus Nijhoff Publishing Boston pp 1-25, 1988), i.e., calves, lambs, or piglets that are commonly, persistently infected for life often do not thrive well, are prone to immunosuppression and (sub)clinical disease (such as mucosal disease with BVDV (Brownlie, J. Arch. Virol. [Suppl. 3]:73-96, 1991)) and, last but not least, are a continuing source of infection for the rest of the population.